The present invention relates to an electronic postal meter and more particularly to an electronic meter which is highly secure from tampering involving the data processing capabilities of the meter.
Postal meters in use today are, almost universally, mechanical devices in which postage values are set, printed, and accounted for by means of mechanical assemblies such as linkages and registers. Such meters include a mechanical ascending register which provides a record of the amount of postage printed over the life of the meter. The meter also includes a mechanical descending register which provides a record of the amount of postage remaining for use in the meter. To prevent tampering with the critical functions of such mechanical meters, a number of different mechanical interlocks have been used. Such interlocks prevent a user from printing postage amounts without changing the contents of the ascending and descending registers. Similarly, such interlocks make it nearly impossible for a user, without leaving telltale signs, to reset the descending register himself to "recharge" the postal meter.
Electronic postal meters have been developed. In such meters, a computer device such as a microprocessor may calculate postage amounts and cause an electrically driven printer to be set to the proper postage amount. All data, including critical accounting data, is stored in electrical format in memory units.
The advantages of electronic postal meters are known. Such meters, having fewer mechanical parts, should last longer and prove more reliable than mechanical meters. Furthermore, electronic postal meters are extremely versatile devices which may perform functions that cannot practically be performed in a purely mechanical meter. For example, an electronic postal meter may include logic circuitry for determining the destination zone of a package given the zip code of the point of origin and the zip code of the point of destination. Moreover, such meters can generally be more readily changed to accommodate changes in the postal regulations or rates. Also, such meters are generally capable of performing at high speeds, a necessity for high volume mailing operations.
While electronic postal meters have many advantages, they also present certain problems which had already been solved in the widely-used mechanical postal meters. The use of electronics to perform the necessary meter functions renders obsolete many of the mechanical interlocks formerly developed to prevent tempering with the meter contents. Naturally, this increases the risk that a user knowledgeable in the electronic technologies employed in a postal meter may find a way to print postage amounts without these amounts being registered in the descending or ascending registers. Similarly, a knowledgeable and unscrupulous user may attempt to develop a method for "recharging" the meter without the normally necessary payment to the Post Office.
Another problem which can arise with electronic postal meters is that their proper operation depends upon the proper functioning of many components which cannot be readily inspected. For the most part, these components are "binary" in nature; that is, their output is either on or off. A failed component may, unless noticed, provide an unchanging output which would be interpreted erroneously by the microprocessor.
Still another problem with electronic postal meters is that such meters will not necessarily be disabled upon a malfunction or failure in a particular section or upon the occurrence of certain events. The meter will continue to function, albeit perhaps improperly, until instructed to stop.